[jsmith45]

Installing per user was an MSI feature that I'm pretty sure predates UAC.

The idea was not about installing without user's permission, but was entirely about allowing normal (non-admin) users to be able to install software, which would obviously be limited to their account only.

This means it needs to install to part of the filesystem where a non-admin user has write permissions. There are only two possible places. The user's profile, or the Programdata folder. Installing to the latter would have problems where one user could maliciously modify a program another installed (or issues if two users installed different versions of the same program), so the user's own profile is where it must go.

This concept is nothing new, users could long compile and install software into their home directories on unix platforms.